Projection screens come in two basic forms, front projection and rear projection screens. The front projection screen has the light source on the same side from which image is viewed. The rear projection screen has the projection source on the opposite side from which the image is viewed.
Front projection screens have at least one layer which is opaque or reflective so that projected light returns to the viewer, while rear projection screens are either transparent or translucent so that projected light travels through the screen to the viewer. By way of example, a flat, white wall would be the simplest front projection screen, while a traditional view camera can be considered a simple rear projector. The present invention is directed to embodiments for both front and rear projection screens.
The camera obscura was the earliest known front projection screen, consisting of a flat white surface that required an absolutely pitch dark room. Since then countless fabrics, paints, plastics and metals have been used in various combinations as projection surfaces for viewing light projected images.
The original rear projection or "diffusion" screen was comprised of translucent ground glass and was first used to compose and focus the picture on the back of view cameras. The state of the art progressed to sprayed coatings applied to the face of a glass or clear plastic sheet, which was then installed in a framed opening in an enclosure.
More recently, pigments that were previously used to spray onto glass or clear plastic sheets have been mixed directly with the formative ingredients before the sheets are cast. These methods, currently used in the art, are complicated and difficult to perform, requiring expensive and sophisticated techniques.
The difficulties arise because even distribution of the pigments in the sheet matrix, whether cast or sprayed, is extremely hard to achieve. Even disbursement of pigment is a critical requirement because rear projection screens must have an evenly translucent quality over it's entire surface to form a clear image for the viewer. Unlike front projection screens, rear screens do not have a reflective surface layer and are dependant on the pigments to form suitable images for the viewer.
A further problem associated with prior art screens of the type under consideration is due to the fact that the picture image seen on a conventional screen is produced by projected light. The "bright" portions of the picture are derived primarily from the projected light which passes, for instance, through the transparent portions of slide film, while the "dark" parts of the picture are, in effect, the absence of light caused by the opaque portions of the film which block light from the projector. The degree of darkness in the projection room or theater controls the degree of darkness in the formed image.
Alternatively, where there is more ambient light available, there is a lesser differentiation between dark and light portions. Thus, the richness of the variation between the dark and bright portions of a projected image comes from the differences in the amount of light projected versus the amount of darkness in the viewing room. This is referred to as "contrast."
As a general rule, the simpler the construction of the screen, the more darkness is required for sharper contrast. Any amount of ambient light tends to weaken the contrast of the image on the screen. Thus, when someone admits daylight into a movie theatre by opening a side door, the picture "washes" out because the dark portions now reflect light. In such situations, contrast can be enhanced by adding "blackness" to the screen.
Currently, one way of accomplishing this is by interweaving black and silver fibrous strands to form a front projection screen which contains finely alternating black, dark areas interwoven with silver, reflective areas. For rear projection screens, blackness is added by initially casting a molded screen surface creating subtle, alternating protrusions and depressions. Blackness is then applied only to the protruding areas of the screen surface.
However, these methods, currently used in the art, are complex and difficult to perform, requiring expensive and sophisticated techniques. The difficulties arise because even distribution of the dark areas whether interwoven or applied to cast-molded screens is extremely hard to achieve.